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Joined: Oct 2007
Posts: 56
SP4RX Offline OP
The new construction building I'm working on had a problem the other night. The concrete finishers were working late and were the only ones left on-site so some of the details are unclear.

First off, the site's electricity is supplied by a diesel powered, 3-phase, 208/120V Y generator. There is a piece of cab-tire feeding a temporary panel inside the building. The panel is a single phase FPE.

Now to the problem. What the site super told me happened is that the concrete finishers had 2 portable 1000 W M.H. fixtures operating on a single extension cord plugged into a 15 A cct. The GFCI plug that the extension cord was plugged into is at the temporary panel. Somehow the plug managed to melt down and catch fire. The fire was contained to the single plug, and completely destroyed 1/2 of the could look right through where that half of the receptacle was and see the back of the box.

I show up yesterday morning and the super tells me what had happened the previous night, so I check out the damage. The first thing I did was to use my voltage sensor to be sure that the conductor feeding the destroyed plug is dead. However, I couldn't get the sensor within 6 or 8 inches of the plug or panel without it beeping and flashing.
This got me wondering because that's kind of the way my voltage sensor reacts to 600/347V.

I pulled off the panel cover to take some meter readings and things get really weird. While I was removing the panel cover I could see slight arcing a couple times in the burnt-out box. Once the cover was off I could clearly see that the wire from the fire-plug went to a breaker that had been turned off. So then I'm wondering what's causing the arcing, the neutral or ground? I metered things out and here's what was measured:
A phase - Neutral--------------120V
B phase - Neutral--------------120V
A phase - B phase--------------208V
so far so good...but wait
A phase - Ground---------------140V
B phase - Ground---------------333V
Neutral - Ground---------------266V

I guess that explains the arcing I saw...the neutral was arcing to ground.

So how is this possible?

The generator has a digital display that displayed normal voltages (for L-N and L-L) and frequency.

There was a ground wire going from the generator to a ground rod pounded at the generator. I checked the ground connections at the generator and the rod and they were good.

I metered the cab-tire to see if there were any opens/shorts but it was good.

The only thing I could see that might cause the situation was a possible problem with the generator so I recommended that the super have a service technician come out and take a look at it. He decided to have it picked up and taken off-site yesterday. Today the supplier calls and says that the generator is fine. What gives??

I'm completely stumped and really want to know what caused this to happen.

If any of you have any input/suggestions/ideas, I'm all ears.

Thanks for listening.


Joined: Jul 2007
Posts: 1,335
The generator lost its main bond is my first thought but what about the C phase readings?


"Live Awesome!" - Kevin Carosa
Joined: Sep 2004
Posts: 68
The problem could be in the cab-tire. At first glance it souncs like I know what I'm talking about, except I have no clue what cab-tire is.

Joined: Feb 2003
Posts: 939
Is that generator is multivoltage / multiphase unit ??

if so.,, it possiblty that one phase at the generator got mess up ?? this can happend with combation zigzag/wye connection.


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

Joined: May 2003
Posts: 74
Uh what's a cab-tire?

Joined: May 2006
Posts: 20
cabtire is slang for rubber insulated cord (type S, SO etc).

Joined: Oct 2007
Posts: 56
SP4RX Offline OP
Quote: "The problem could be in the cab-tire. At first glance it souncs like I know what I'm talking about, except I have no clue what cab-tire is."

LOL I would expect nothing less from somebody who chooses "homer" as a nick. smile

Everybody around here refers to "flexible cable having a heavy rubber or neoprene outer sheathing" as cab-tire, don't ask me why.

As to the generator problem,Sparkyinak nailed neutral bonding at the generator. I was thinking that the problem would be more along the lines of what frenchy was getting at, but at least it was a simple fix. The repair place called the site super yesterday and told him they had inspected the gen-set and it was working properly, and they would return it. When I got to the site this morning the generator was back. I opened the cover on the termination cabinet to re-attach the cab-tire and the first thing I noticed was a bonding jumper going from the neutral bus to the ground lug. I know this jumper was not there two days ago because I looked for one. I mistakenly assumed that the bonding had been done inside the unit somewhere.

I'm really surprised how much of a difference can be made by removing the neutral bond, I never would have guessed it would be this dramatic.

This generator has been on-site for about 6 weeks in this condition, I'm surprised nothing bad has happened.

Thanks for the responses!


p.s. the gen-set was feeding a single phase panel so C phase was not being used

Joined: Jul 2007
Posts: 85
It seems to me even though you solved the main problem there is more trouble here. a three phase genset, feeding a single phase panel. Not to mention the Fpe panel, you are asking for a fire. you're lucky it was isolated to one receptacle

Joined: Oct 2007
Posts: 56
SP4RX Offline OP
I'm not sure I follow...where is the problem? If the generator is only feeding 2 phases to the panel, the panel operates at 120/208. The panel is rated for 120/240. The third phase never leaves the load side of the breaker at the generator. The only thing I wondered about when first seeing this setup was how the imbalanced electrical load (2 phases out of 3) would affect the mechanical load on the diesel engine.

We provide all of our sites with temporary panels, and they're always 120/240. This GC bought a new generator and for some reason decided to buy a 3 phase model, although I can't figure out why. I think what happened here is that the temp panel was already built and on-site when the new genny showed up...our guy went to the site and just connected the cable from the panel to the new unit.

If you think there may be issues with this setup I would really like to know what concerns you so I can prevent any further problems.

Joined: Jul 2007
Posts: 1,335
running a three phase genset on only 2 phases is not the healthiest for the genset. It is like running a 3 cylinder engine on only 2. It is not as bad on a wye as it is on a delta (so I have been told). Other factors include the size of the genset and the loads on it.

"Live Awesome!" - Kevin Carosa
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